Showing papers on "Microphone array published in 1984"

Acoustic near-field properties associated with broadband shock noise

Abstract: Shock noise associated with unheated supersonic jets was investigated using a near-field microphone array and a single-sensor wedge-shaped hot-film probe. Both over- and underexpanded cases were investigated using Mach 1.45 and 1.99 convergent-divergent nozzles. Correlation measurements through each shock cell of a single underexpanded case with the Mach 1.45 nozzle were obtained between the hot-film probe and the microphone array. The results of the hot-film/near-field microphone correlations show general agreement with certain theoretical models as to the location for shock noise production, and provide evidence for the existence of some large-scale flow structure that collectively interacts and phases the motion of the downstream shocks. The nearfield microphone correlations demonstrate that downstream shocks dominate shock noise production and suggest the existence of a Doppler effect in the near field of the sources. In addition, broadband shock noise is found to propagate at small angles to the jet axis.

Adaptive microphone‐array system for noise reduction

Abstract: A new adaptive microphone‐array system for noise reduction (AMNOR system) is introduced. The AMNOR system detects the arrival directions of noise and forms a directional pattern possessing low sensitivities to those directions using a digital filtering technique. With pre‐informed data on the direction of desired signal source, the AMNOR system learns the noise arrival directions previously. Optimum filter coefficients are determined based on a new criterion, whose concept is to minimize output noise power while maintaining the degradation in the frequency response to the desired signal below the predetermined value. An algorithm for calculating the optimum filter is proposed. To confirm the effectiveness of the AMNOR system, experiments on the noise reduction processing were carried out in a room with a reverberation time of 0.4 s. A small circular microphone array (radius of 8.5 cm) with four microphone elements and four FIR filters with 16 taps were used. The convergence time of the algorithm at this experimental condition was 0.3 s. Improvement in SNR as a result of the AMNOR system by more than 15 dB in the frequency range of 300–3300 Hz. Superiority over the conventional LMS criterion for noise reduction of speech signals was also confirmed by subjective preference tests.

Directional microphone device

Abstract: PURPOSE:To obtain an excellent directivity, by switching an element constitution in accordance with a direction of a main lobe of an array consisting of a pair of microphone elements whose main shaft is inclined, respectively, by an angle in less than a specified angle, in a clockwise direction and in a counterclockwise direction. CONSTITUTION:When varying the direction of a main lobe (front lobe) of a microphone array MA to an angle by less than 90 deg. counterclockwise from the front direction, microphone elements A1l-Anl are used, and when varying said direction to an angle by less than 90 deg. clockwise, microphone elements A1gamma- Angamma are used. The element A1l and A1gamma constitute a pair of elements, and each element pair is placed at prescribed intervals on the same plane. The element A1l-Anl are provided by inclining them by an angle theta in the clockwise direction, and also the elements A1gamma-Angamma are provided by inclining them by an angle theta in the counterclockwise direction. A controller CR is adjusted, and a control signal generating circuit CSG executes control of a delay quantity of delaying circuits D1-Dn-1 and switching of changeover switches SW1-SWn, by which the excellent directivity is obtained.

Microphone device having directivity

Abstract: PURPOSE:To eliminate a backside lobe by changing directivity to make the backside lobe insensible when the direction of the front lobe of a microphone array is changed CONSTITUTION:The directivity of the microphone array is varied by using pressure microphones A1v-Anv and velocity microphones A1p-Anp as unit microphone elements for the constitution of the microphone array and changing the rate of mixture of the output signals of both microphones As the direction of the front lobe of the microphone array is changed over the required front range of the microphone array, the back side lobe produced at the position specular symmetrical to the front lobe is made insensible in the microphone element units A1-An by changing the directivity of the unit microphone elements A1-An to eliminate the backside lobe

A new approach to transducer design applied to a foil electret acoustic antenna

Abstract: A new approach to transducer design is introduced in which a transducer is viewed as a continuum of infinitesimal, locally reacting elements. The transducer response is the coherent sum of the responses of the infinitesimal elements. Hence variations in transducer response characteristics will be produced by variations in the relative contribution of each element. It is thus possible to produce designed transducer characteristics through selection of a suitable local sensitivity shading function. This new approach is used to design a foil electret acoustic antenna which achieves with a single transducer direction characteristics similar to these of a linear microphone array. Five methods for accomplishing sensitivity variation in the electret microphone have been identified: variation of active width, variation of charge density, variation of actual airgap, variation of foil thickness, and variation of effective airgap. It is shown that compared to a linear microphone array, the single transducer acoustic...

Wind noise reduction using a two microphone array

Abstract: The effectiveness of a method for wind noise reduction by cross correlation of the outputs of two vertically spaced microphones was studied. Average wind noise reduction was calculated for several microphone spacings and sample lengths; improvement in noise level on the order of 8 dB was achieved for 1‐s samples with microphones spaced 31 cm apart, with better performance for longer samples and greater spacings. Correlation of acoustic signals was also investigated.

Measurements of the Low Wavenumber Wall Pressure Spectral Density during Transition on a Flat Plate.

Abstract: : Experimental measurements of the low-Mach number wall pressure spectra density in the transition region are currently carried out in our laboratory The six element (B&K Model No 4144) microphone array used by Farabee and Geib was used as the wavenumber filtering apparatus and flush mounted on an open test section identical to that used by Jameson A technique of conditional sampling on signals was used to determine the characteristics of the transitional flow The frequency spectrum of the microphone array output was computed for uniform shading, Chebyshev shading and binomial shading A detailed update on our progress is enclosed The preliminary results indicate that there is no significant measurable difference in the low wavenumber wall pressure spectrum density for a transitional boundary layer as compared to a fully turbulent boundary layer (Author)